This invention relates to integral tensile-strength reinforcing materials applied to concrete structures.
The tensile strength of concrete is approximately one tenth that of its compression strength. For this reason, concrete structures subjected to bending or deflection, such as beams, roofs, columns, piling, and buried pipe must be reinforced by a material that increases its tensile strength.
The material most commonly used previously to reinforce concrete is carbon steel. Among the advantages of using carbon steel as a concrete reinforcement material are its low cost, its ready availability, its predictable physical properties and its long history of use and approval by building code committees.
However, in many applications serious problems have been encountered with the use of these carbon steel reinforcements. Corrosion of carbon steel reinforcing members has caused the deterioration of concrete bridge decks, concrete pipe and other concrete structures. For example, a primary cause of bridge deck deterioration is the cyclic freeze-thaw exposures and the reinforcing steel corrosion caused by the extensive use of de-icing salts.
Practical realities of the concrete formation process can exacerbate steel corrosion problems. For example, due to the shortage of fresh salt-free water in certain regions of the world, steel-reinforced concrete structures have frequently used saltwater in the concrete mix. When sea water was utilized in the concrete mix used to build reinforced concrete structures in Saudi Arabia, the resulting high internal chloride level of the concrete produced extensive corrosion of the steel reinforcement within the concrete as well as cracking, delamination and spalling of the concrete.
The steel bar and wire materials used to form and reinforce concrete are generally placed inside rather than outside the concrete structure, for several reasons. First, it is difficult and expensive to bond or otherwise attach steel reinforcement members to the exterior of concrete structures subjected to beam loads. Second, encasing the carbon steel reinforcement members within the alkaline concrete material protects the steel from corrosion due to acidic water.
However, the placement of steel reinforcements within the concrete structures they reinforce presents numerous drawbacks. In a typical concrete beam, its bottom exterior surface bears the greatest tensile load. Accordingly, placement of the steel reinforcement within the concrete beam fails to support the beam at its weakest point. Internally placed steel reinforcements do not enclose the outer surface of the concrete, and thus provide no protection for the outer surface from water intrusion or leaking. Similarly, internally placed steel reinforcements do not prevent concrete from spalling or breaking loose in crisis conditions such as an earthquake. Furthermore, steel reinforcements placed within a reinforced concrete structure are hidden from view and are thus difficult and expensive to inspect.
In order to avoid problems with corrosion, makers of concrete structures have turned to nonmetallic materials as alternatives to carbon steel reinforcements. For example, steel reinforcing bars can be replaced by pultruded bars of fiber-reinforced plastic ("FRP") or filament-wound FRP tubular structures, or steel mesh can be replaced by FRP grating or screens. These materials, however, are used as internal reinforcements. Thus, their use does not alleviate the problems described above found with all internally placed reinforcements.
Corrosion-resistant stainless steel fibers or alkaline-resistant fiberglass fibers may be intermixed or otherwise placed within concrete before it hardens in order to increase the tensile strength of the concrete. However, this method fails to protect the outer surfaces of the concrete structures it reinforces, and further requires high cost and complex mixing procedures in order to provide uniform dispersion of the reinforcing fibers within the concrete.
Certain externally-mounted structures have also been explored as alternative means to reinforce concrete. For example, paper-thin polymeric composite laminates, made in the form of sheets, can be bonded to the exterior surface of a dry concrete structure. Such composite laminates, made from continuous carbon fibers and a prepreg epoxy resin, have been used to reinforce or repair concrete bridge decks and concrete walls. In California, polymeric composite materials containing continuous filament reinforcements have been used after earthquakes to reinforce fully cured concrete column structures that support automotive highways. These composite sheet reinforcements are usually bonded to a dry concrete surface with a thin layer of epoxy resin adhesive.
These external surface concrete reinforcements are expensive to make and apply and their reinforcing strength depends upon the bond strength between the composite laminate and the concrete surface material. Because the composite laminates are not bonded to the concrete until after the concrete has already been cured, no truly intimate bond between the laminate and the concrete can be made. Furthermore, the bond strength which can be established between the laminates and the concrete is vulnerable to the low peel strength characterizing epoxy adhesives. Long term exposure to weathering and severe temperature changes can also cause the thin composite sheet to delaminate from the concrete structure.
Beyond reinforcements, external structures used in the creation of concrete structures include shaping forms into which wet concrete can be poured and maintained in a desired shape until it dries. If external conditions are such that ice forms in the concrete while it dries, the concrete can lose nearly half its potential design strength, even though cement hydration can be reestablished upon re-warming the frozen concrete. Keeping concrete warm or using accelerators to reduce the curing time increases the cost of the concreting job. Thus, to control the temperature of the concrete as it dries, these forms have been made of thermally insulative materials. However, these forms have not also served to reinforce the completed concrete structure or permanently seal its outer surface.
Shaping forms are used in the formation of centrifugally cast concrete pipes. One characteristic of these pipes has been that, due to unavoidable variations in the quantity of concrete placed within the rotating form, the pipe cannot be made to possess identical internal diameters.
Conventional concrete pipe liners, such as those cast within the concrete, are usually made of flexible sheets of thermoplastic materials that do not increase the structural strength of the pipe. The interior of conventional concrete sewer pipe is commonly protected from the corrosive effects of the sulfuric acid produced by hydrogen sulfide in sewer gas, by cast-in-place pipe liners made of poly vinyl chloride. These pipe liners sometimes have protrusions which are pushed into the wet concrete in order to anchor the liner to the concrete. However, these protrusions are not formed to be integral structural constituents of the concrete pipe.
One particular method used to attach pipe liners to concrete pipes has been to extend circumferentially spaced extruded tee shapes longitudinally within the pipe wall. A downside of this method is that the concrete is weakened in direct proportion to the depth of the plastic anchor tee. Such "tee locks" provide longitudinal grooves that serve as built-in stress-risers that can produce fractures in the concrete pipe structure when the pipe is shifted during earthquake or other soil motion events.
One attempt to create a concrete liner having protrusions which can be made internal structural constituents of the concrete is known. To create this liner, a laminate surface was coated with a bonding resin, and rock aggregate particles were sprinkled upon the resin. The rock aggregate particles were then embedded in fresh concrete. However, it was found that the resulting bond strength for the liner was limited to the tensile strength of the hardened resin present between the bottom of the rock particle and the laminate surface with which it was in contact. This particle bond strength was found to be less than the tensile strength of either the rock particles or the concrete. For this reason, such aggregate covered laminates were deemed not suitable as concrete reinforcement constituents.